As ADM1 code >>
"0" means not attributed, unknown... the creator of the feature omitted or did not know the ADM1.
"00" means general or countrywide... the feature cannot be attributed to a single ADM1.
The feature refers to a region (e.g. Mariana Islands) or a countrywide phenomenon (desert, rain forest...)

As ADM2 code >>
...under an ADM1 it is not meaningful, without ADM1 it is false.
It is not possible to create a single ADM2 as "00".
These "00" were probably introduced from another data base...